The invention relates to improved techniques for modulating carrier signals, and more particularly to techniques for modulating a carrier signal independently of the amplitude, frequency, and phase of the carrier signal to cause each half-cycle of the carrier signal to represent the logic states of a plurality of digital bits, and to techniques for demodulating such a carrier signal, and to techniques for generating frame markers therefore, generally in accordance with the teachings of above-described application Ser. No. 590,281.
A variety of modulation techniques are very well known, including amplitude modulation, frequency modulation, and phase modulation. Although it is possible to transmit digtal pulses directly along a communications medium, especially a wire or optical medium, it ususally is not practical to transmit large amounts of data directly in digital form via a wireles communication link. Consequently, digital data is "encoded" into high frequency modulation carrier signals using various pulse coded modulation (PCM) techniques, such as high frequency shift keying (FSK) techniques. All of these techniques require at least several cycles of the carrier signal for each digital bit transmitted.
Such digital data, encoded in analog form into modulated high frequency sinusoidal carrier signals by well-known modulation techniques, often is received by a receiver and stored (as received) on magnetic tape for archival purposes, and when it is necessary to be retrieved from the magnetic tape, the modulated signals are played back from the tape, fed into a demodulator, and then converted to digital form.
The state-of-the-art in transmission of digital data and modulation techniques is believed to be indicated by U.S. Pat. No. 3,993,862, Re-issue No. 30,182, U.S. Pat. Nos. 4,106,007, 4,227,250, 4,236,248, 4,339,823, 3,764,743, 3,821,481, and 3,755,739.
It would be desirable to have a technique for inexpensively modulating a carrier signal independently of its frequency, amplitude, and phase. It would also be desirable to have a technique for encoding or modulating a carrier signal to contain information which can be faithfully demodulated, despite considerable distortion imposed upon the modulated carrier signal by the communications channel. It would also be desirable to have a technique for encoding or modulating a carrier signal so that every half-cycle of the carrier signal contains information representative of the logic states of a plurality of digital bits. It would also be desirable to have a more economical, higher density storage of digital information than is presently available.
Although the techniques and circuits disclosed in above-identified parent application Ser. No. 590,281 appear to represent a breakthrough in the modulation/demodulation art, further research and experimentation has led to improved techniques for producing the modulated waveforms described therein and for effectuating storage of and retrieval of the modulated signals from magnetic media.
In copending application Ser. No. 590,281, the described invention provides a system and method for producing a signal having the form of a modulated carrier signal wherein in each data-carrying transition, a midrange level or a midrange ratio associated with two characteristics of that transition represents the data carried by that transition. The midrange ratio can be the ratio of the slope of one portion of the transition to the sum of two slopes of the transition, or it can be the ratio of the level of a midrange, relatively horizontal portion of the transition to the peak-to-peak level of the transition. The peaks of the modulated carrier signal can carry clocking information useful in demodulating the modulated carrier signal.
In one described embodiment of the invention described in copending application Ser. No. 590,281, the carrier signal initially is sinusoidal. The amplitude of the relatively horizontal modulation-produced level between successive positive and negative peaks is a predetermined proportion of the peak-to-peak amplitude of the carrier signal. The ratio of the slope of the leading portion of the transition to the sum of the slopes of the leading and trailing portions of the transitions also represents the same ratio. Demodulation is accomplished in one of two basic ways, either by measuring the slopes, or derivatives of the leading and trailing portions of each transition and obtaining the ratio of one of the two slopes to the sum of the two slopes, or by sampling the amplitude of the modulated signal at the beginning of the first transition, at the relatively level portion of the transition and at the end of the transition, and then taking the ratio of the intermediate level to the peak-to-peak amplitude to recover a quantity corresponding to the modulating data level (of the input signal). In a described embodiment of the parent application, a sinusoidal carrier signal is squared, and even numbered cycles of the squared carrier signal are gated by an analog switch to the input of a first analog multiplier. Odd numbered cycles of the squared carier signal are inverted and gated by an analog switch to an input of a second analog multiplier. The outputs of the two analog multipliers are resistively summed. The second input of each of the two analog multipliers is varied in accordance with the modulation data level of the input signal so that the total value of the signals applied to the second inputs of the two analog multipliers is equal to unity. The ouput signal produced by summing the outputs of the two analog multipliers is the modulated carrier signal, each transition thereof having an upper and lower portion separated by an intermediate, relatively level portion, the slopes and intermediate level representing the modulating data. In one described embodiment, the modulating data is produced by performing a digital-to-analog conversion of a plurality of N digital bits for each transition of the carrier signal to be modulated, in order to produce one of 2.sup.N discrete levels as the value of the intermediate level of that transition.
Demodulation of the modulated carrier signal, or a replica therof arriving at the opposite end of a communication channel, is differentiated by a differentiating circuit that separates derivative pulses representing the slopes of negative-going transitions and positive-going transitions. Each transition produces two derivative pulses, the peak amplitudes of which are sampled by peak sense and hold circuits. The outputs of the peak sense and hold circuits are summed to produce a signal representative of the peak-to-peak amplitude of the modulated carrier signal at the time of the present transition. A signal representing the sum of the derivative peaks and a signal representing one of the derivative peaks are input to an analog divider which computes a signal that is representative of the ratio of one of the slopes of a particular transition to the sum of the slopes thereof and which therefore represents the present value of the modulating or input signal. This value is converted by means of an analog-to-digital converter circuit back to the level of the original N digital bits. Since the position of the peaks of the modulated carrier signal are detected with a peak sense and hold circuit, even if the peak is being frequency modulated or amplitude modulated, its occurrence is detected by the demodulator circuit and the data represented by the midrange ratios is recovered.
The presence of a transition without an intermediate level is interpreted as a frame marker. The encoding circuit produces various such frame markers causing direct full range transitions within a half-cycle of the carrier signal, holding the carrier signal at a maximum or minimum level for a full half-cycle of the carrier signal, and/or causing reverse transitions from a maximum or minimum level of the carrier signal to an intermediate level and back during a half-cycle of the carrier signal.
In a described embodiment in the parent application, the encoded analog signal can be stored on magnetic tape (and later played back and fed to the demodulator) to thereby produce an extremely high density of encoding analog or digital information on magnetic tape or other medium. The amount of information per cycle of the modulated carrier signal is many times that of conventional modulation techniques for analog information or for digitally encoded information in analog form.
In accordance with another embodiment described in copending application Ser. No. 590,281, a system and method are provided for producing a signal in the form of a carrier signal of a certain peak-to-peak amplitude and frequency that is comprised of alternating amplitude-modulated half-cycles of exactly twice the carrier frequency. Information is carried in pairs of these half-cycles which are joined internally by a 180 degree phase flip. The sum of the amplitudes of the half cyclces in each pair is constant and equal to the peak-to-peak amplitude of the carrier. Each pair is joined without a phase flip to adjacent pairs. The pairs are of two kinds, rising and falling. Each such transition carries a multilevel group of bits of information in the amplitude ratio between flip-to-peak and peak-to-peak levels. Peaks between pairs carry sample clock information.
An alternate embodiment described in application Ser. No. 590,281 involves the digital generation and recovery of nonsinusoidal digital AUDEL pairs. One simple digital modulation process produces a sequence of "square" levels alternating between high, midrange, low, midrange, high levels, etc., where the highs are equal to each other and greater than any midrange value, and the lows are equal to each other and less than any midrange value. Refinements of this digital process utilize more samples per carrier cycle and analog filter techniques to more closely approximate the desired sinusoidal modulation with fewer high frequency components. For both analog and digital implementations of the technique, a full range transition in a single step, either high-to-low or low-to-high, is interpreted as a frame marker; a transition from a positive or negative peak level to a midlevel and back to that peak level also is interpreted as a frame marker. Holding the carrier signal at either a positive or a negative peak level for an entire half-cycle of the carrier signal also is interpreted as a frame marker.
The signal at the decoder is first passed through a frequency spectrum restoration circuit to recreate the original form to each cycle. Then clock information (i.e., digital sample clock information) is recovered by phase locking a synchronous oscillator to the detected peaks of the modulated carrier signal. All registers in this circuit are updated on the leading edge of the sample clock. All data in this circuit is passed through ratio logic circuitry and threshold and partition logic circuitry during each sample clock cycle. Maxima and minima from the previous cycle are held in registers for presentation to the ratio logic circuitry. The analog peak detector circuitry determines whether the current sample is a high peak, a low peak, or a midrange datum, and generates appropriate control signals. A "previous value register" and a comparator performs a subtraction and magnitude comparison to determine if the current sample is a frame marker. New "data" is presented at an output port along with a flag signal. Any frame marker signal that occurs is produced at a separate digital output.
This mostly digital embodiment of the invention described in copending application Ser. No. 590,281 is less bandwidth-efficient and less noise resistant than other embodiments that fully utilize analog peak slope and phase transition detectors. However, this "digital" embodiment is both simpler and less expensive than the "analog" implementations of the invention, and is very amenable to digital large scale integration.